All four players are now protected from being selected by another club in the Rule 5 Draft, which is set to be held on Dec. 12 at Baseball's Winter Meetings in Orlando, Fla.

Additionally, the Twins outrighted right-hander B.J. Hermsen off the 40-man roster to Triple-A Rochester, so they now have 39 players on their Major League roster.

Kepler, ranked as the club's No. 8 prospect by MLB.com, hit .237/.312/.424 with nine home runs and 11 doubles in 61 games at Class A Cedar Rapids. He most recently played for Glendale in the Arizona Fall League, where he hit .234 with five doubles and four RBIs in 18 games. The 20-year-old was originally signed by the Twins as an amateur free agent out of Germany in 2009.

Polanco, who is ranked right behind Kepler as Minnesota's No. 9 prospect, had an impressive year at Cedar Rapids, hitting .308/.362/.452 with five homers, 32 doubles and 10 triples in 115 games. The 20-year-old played 57 games at second base and 49 games at shortstop but is projected to be a second baseman moving forward. The Dominican Republic native signed with the Twins as an amateur free agent in '09.

Vargas, 23, hit .267/.344/.468 with 19 homers and 33 doubles in 125 games at Class A Advanced Fort Myers. The Puerto Rico native was originally signed by the Twins in '09.

Darnell, 24, posted a combined 3.22 ERA with 120 strikeouts and 45 walks in 153 2/3 innings between Double-A New Britain and Triple-A Rochester. He fared better at Double-A with a 2.61 ERA in 15 starts compared to a 4.26 ERA in 12 appearances (11 starts) in Triple-A. He was originally selected by the Twins in the sixth round of the 2010 First-Year Player Draft.

Right-handed reliever A.J. Achter was a notable player not added to the 40-man roster after posting a 2.54 ERA with 56 strikeouts in 60 1/3 innings between New Britain and Rochester. Achter, 25, is now eligible to be selected in the Rule 5 Draft, as he was originally picked by the Twins in the 46th round of the '10 Draft.

Players first signed at age 18 must be added to 40-man rosters within five years or they become eligible to be drafted by other organizations through the Rule 5 process. Players signed at 19 years or older have to be protected within four years. Clubs pay $50,000 to select a player in the Major League phase of the Rule 5 Draft. If that player doesn't stay on the 25-man roster for the full season, he must be offered back to his former team for $25,000.

In addition, the Twins announced they agreed to terms with 11 players on Minor League contracts with invitations to Major League camp: right-handed relievers Deolis Guerra and Lester Oliveros; left-handed reliever Aaron Thompson; outfielders Jermaine Mitchell, Chris Rahl and Wilkin Ramirez; infielders Jason Bartlett, James Beresford, Doug Bernier and Deibinson Romero; and catcher Dan Rohlfing. Right-handed pitcher Virgil Vasquez was also signed to a Minor League contract.